Mate default theme has the scrollbar set pretty nice, but I still like to tweak it a bit. This is somethig I don’t want to do everytime I switch themes. So here’s a way to tweak the scrollbar that will work for all the gtk3 themes.

NOTE: This will change most, but not all windows. Example: No effect on Syaptic Package Manager.

Create a new file and name it “gtk.css”.

Place this file in your home folder at the location /.config/gtk-3.0/gtk.css

Note:
This tweak should work on all default themes. It was created using GreenLaguna, so milage may vary.

Scrollbar/buttons/slider tweaks will need to be added to (home directory) ~/.config/gtk-3.0/gtk.css (the /gtk.css file will need to be created at that location) and this is a per user modification that will not work system wide. Currently I cannot add a custom theme system wide using root access to /themes or user themes to /.themes. The appearance GUI just will not see them.